Polychlorinated biphenyls (PCBs) are persistent and environmentally hazardous compounds that occur in many soils and sediments as a result of past industrial activity. PCBs are typically mixtures of isomers of trichlorobiphenyl, tetrachlorobiphenyl, pentachlorobiphenyl, dichlorobiphenyl and hexachlorobiphenyl, but may contain other polychlorobiphenyls. Up until the early 1970's PCBs were used in a wide range of applications due to their unique blend of fire resistance, thermal and oxidative stability, electrical characteristics, solvency, inertness and liquid range. Among their most important uses were as a dielectric medium in transformers, either alone or in blends with other materials such as trichlorobenzene; as the dielectric impregnating medium in capacitors; as plasticizers; as ingredients in lacquers, paints and varnishes and adhesives; as water proofing compounds in various types of coatings; as lubricants or lubricant additives under extreme conditions; as heat transfer fluids; as fire resistant hydraulic fluids; as vacuum pump fluids; and as air compressor lubricants. Their largest application was in the electrical industry as a dielectric medium in transformers and capacitors.
However, in the late '60's and early '70's, it was discovered that PCBs have a major potential for environmental contamination due to their extremely slow biodegradation rates. The persistence and toxicity of PCBs have prompted governmental action restricting their use and application. The current laws and regulations contain provisions for discontinuance of their use and for their eventual disposal.
Even trace amounts of PCBs are considered undesirable. Notwithstanding their chemical inertness PCB compounds have been found to or been suspected of being a carcinogen, a development toxicant, gastrointestinal or liver toxicant, a neurotoxicant, and reproductive toxicant, and respiratory toxicant, and a skin or sense organ toxicant. PCBs rank among the more hazardous chemicals, for even in small amount, PCBs displays toxicity to both humans and wildlife.
Large deposits of PCB contaminated soils and sediment remain in many areas of the country. These deposits continue to release PCBs into the environment. Of special concern are PCB contaminated sediment deposits at the bottom of water bodies such as rivers, rivers, lakes and waterways. These sediments, even those containing only low concentrations of PCBs, pose human health and ecological risks from low-level food chain interactions and impair water quality due to the release PCBs into the water column. Sport and commercial fisheries on many water bodies have closed due to fishing restrictions associated with PCB contamination in fish. The problem is vast. Numerous Superfund sites involve PCB contaminated sediments. Some of the most well known include the Hudson River, the Passaic River, the New Bedford Harbor, and the Fox River. The size of some of these sites is almost overwhelming; nearly 200 miles of the Hudson River is a designated Superfund site and the Fox River in the State of Wisconsin contains over 10 million cubic yards of PCB contaminated sediments in over 40 miles of riverbed. The problem extends well beyond the Great Lakes and its tributaries, however. PCB contaminated sediment sites exist in all areas of the US and are especially prevalent in coastal areas.
Currently methods used to remediate PCB-contaminated sediments can be classed in two basic groups, (1) removal and treatment of the sediments, and (2) capping. Removal and treatment typically involves dredging the sediments and transporting them to a remote site where the sediments and associated carrier water are treated by some physical or chemical process to reduce the PCB concentration or bind the PCBs permanently with the sediment particles. Although several PCB treatment methods have been developed and pilot-tested, for example, incineration, solvent extraction, and vitrification, none have been implemented in a full-scale remedial effort. These methods face a multitude of problems, but logistics, costs, and disposal of residuals are the most difficult to solve. For a site like the Fox River and Hudson River, remediation by removal and treatment (currently planned in both cases) may cost billions of dollars just for rudimentary treatments involving only removal of the contaminated sediments, without consideration of the additional costs and problems associated with of the transportation, treatment, and final disposal of sediment. Thus, most PCB-contaminated sediment projects that have proceeded involving significant quantities of sediment are those in which physical separation was deemed an adequate treatment and the PCB-contaminated sediments were to be buried in either an upland or in-water disposal facility. However, the dredging process itself can be very disruptive to the ecology, and creates a disposal problem for the large volume of treated sediments.
Capping involves depositing stable materials, such as gravel or sand, on top of contaminated sediment. The intention is to isolate the PCB-contaminated sediment from overlying water and thereby prevent PCB migration into the water. While effective in some environments, there are places where this method cannot be applied, due to the currents, hydrology and other factors. In addition, capping is often not desirable because it can radically and permanently alter the ecology of the riverbed. This method is also costly, involving material costs, transportation costs, and the cost for laying the material in the riverbed. Capping is also often viewed as only an interim solution since the PCBs are still in place under the capping layer. Another problem with both removal and capping systems is that, during removal or capping, PCB-laden sediment may be suspended in the water, amplifying the toxic and environmental hazards and increasing the turbidity of the water.
In-situ methods have been used for remediating contaminant-containing soils on land-based sites, which involve surface treatment or well injection with chemicals, bacterial inoculants and gases. However, these methods are not adaptable and are not practical for underwater sediments for many reasons. Accordingly, it is believed that there are no known in-situ methods that are used for remediating underwater sediments on a commercial scale.
An in-situ method has been proposed and tested on a pilot scale at sites in Hamilton Harbour, Ontario. In this method underwater sediments are treated with nutrients to accelerate biodegradation. It has been proposed for the reduction of PAHs, BTXs, TPHs and petroleum hydrocarbons (volatile organics). The method involves an underwater harrow towed behind a boat to till the contaminated sediment and inject it with a chemical oxidant, usually calcium nitrate. Since calcium nitrate is also a nutrient, it must be injected deep into the sediment to prevent it from escaping into the water column and boosting the growth of algae. The harrow is attached to and dragged behind a boat or barge, and includes a series of nozzles, tines and injections ports. Chemicals injected include the oxidants to oxidize any sulphides (which are toxic to bacteria) into non-toxic sulphates and nutrients (phosphorous, nitrogen, carbon) to encourage bacterial proliferation and activity. A flocculating agent is also added along with the chemical as a control for sediment re-suspension as the chemicals are injected.
However, this method by its harrowing of the sediment has the potential of creating a large quantity of suspended silt capable of releasing hazardous chemicals. In addition, PCBs are biologically degraded very slowly even with supplemental nutrients, thus biodegradation cannot be relied upon practically as a sole remediation technique. Possibly for these and other implementation problems, there are no known applications outside of the pilot testing. In summary, this method has only limited applicability and is not applicable to PCBs. In addition, it can contaminate the water body through release of the reagents to the water, and has inherent turbidity and silt suspension problems.